COLD CRYSTALLISATION

No. 18, 1996,p.4009-24 FOURIER TRANSFORM RAMAN STUDY OF THE STRAIN-INDUCED CRYSTALLISATION AND COLD CRYSTALLISATION OF NATURAL RUBBER... 

Figure 1.9 shows an example of detecting a glass transition beneath a cold crystallisation exotherm. The total heat flow corresponds to a conventional DSC experiment. It is not possible from inspection of the distorted peak in this curve to conclude that it is formed from an exotherm (from the crystallisation of PET) superimposed on a glass-rubber transition (from the polycarbonate). The additional signals of MTDSC make this interpretation clear. In this case, the crystallisation acts like a chemical reaction once formed the crystals remain as the temperature increases through the peak. Thus, the process is non-reversing. 

Figure 1.25. MTDSC results for quenched PET showing the peak in the reversing signal that comes from the reorganisation process that occurs after the cold crystallisation (underlying heating rate 2°C/min, period 60 s, amplitude 0.32°C under nitrogen).

The effects due to very slow processes in the sample, however, may not be removed even in the quasi-isothermal MTDSC with data collection after 10 min. The analysis of the slow response of the sample in the glass transition region will be treated in Section 4.3. The decrease in the heat capacity due to cold crystallisation can easily be converted into a plot of the crystallisation kinetics. Additional points for the kinetics plot can be generated at shorter and longer analysis times of the quasi-isothermal runs. The time-scale can easily be adjusted to modulations from 1 min to many hours, limited only by the patience of the operator and the stability of the calorimeter. 

Figure 4.54 is a quantitative quasi-isothermal MTDSC trace for quenched, poorly crystallised PTT. The corresponding semiquantitative MTDSC is depicted in Figure 4.38. The cold crystallisation at about 325 K, the recrystallisation, 450 K, and the small enthalpy relaxation at 320 K are seen to be fully irreversible, and as in PET, the kinetics of the glass transition and the cold crystallisation can be further analysed quantitatively making use of the reversing heat capacity. It is also clear that during the standard DSC measurement, the cold crystallisation never stops completely between the two peaks and considerable errors in the crystallinity may result from choosing a baseline without MTDSC data.

In a separate study, PCL (molecular weight 15,000) was blended with poly(L-lactide) (molecular weight 10,000) by casting from solutions in chloroform and drying under vacuum at 60 °C for 24 h. DSC scans of samples heated to 200 °C and quenched showed that the Tg of poly(L-lactide) overlapped with the Tj of PCL, which rendered detailed analysis of the thermograms and assessment of polymer-polymer miscibility difficult. However, the cold-crystallisation exotherm of pure poly(L-lactide), at 131 °C, was shifted in the blends containing more than 50 wt % poly(L-lactide) to about 95 "C this shift was independent of the poly(L-lactide) content. In addition, the crystallisation rate for PCL was... 

PEKEKK and PEKK can have closely similar structures when crystallised from the melt with a small shift in diffraction peak positions to a lower angle with an increase in keto content - reflecting a change in unit cell parameters [5]. However polyaryletherketoneketone (PEKK) can exhibit a second structural form with face to face phenyl packing. This form is favoured by solvent crystallisation (e.g., in as-polymerised material), by cold crystallisation and by a higher 1,4 to 1,3 ratio. Solvent-crystallised PEK and PEKEKK have been reported to show both polymorphs [5]. The unit cells of seven oriented PAEK were compared by Blundell and Newton [6] who observed that the crystalline density remains almost unchanged at 1.400 g/cm with a standard deviation of 0.006. 

In the author s opinion there is ample evidence for the existence of smaller, less perfect lamellae which form in the constrained amorphous phase between primary lamellae. It is perfectly reasonable that this would be so - since secondary crystallisation occurs in a constrained amorphous phase. Accordingly the second interpretation is favoured. However, it is also true that melting and recrystallisation occur during a heating scan and so the small LTMP may represent more of the lamella size distribution than the small size of the peak would suggest. In cold-crystallised samples there may be a lot more small lamellae than the size of the LTMP indicates (because melting is accompanied by a simultaneous crystallisation exotherm) and indeed the clearest evidence for the melting-recrystallisation interpretation comes from cold-crystallised materials. 

Crystallisation can also occur in the solid state. This is usually associated with semi-crystalline materials that crystallise rather slowly and therefore can be quenched into a glassy solid. A well known example of this type of polymer is poly(ethylene terephthalate). Upon heating to just above its glass transition temperature, crystallisation starts. This is known as cold crystallisation. 

Figure 2.5 PET heated slowly after rapid cooling (solid line) and heated slowly after slow cooling (dashed line). After rapid cooling from the melt the PET remains fully amorphous, so shows a large Tg followed by cold crystallisation on heating. If cooled slowly the PET will partially crystallise, so showing a smaller Tg on heating and no further crystallisation.

Figure 9.8 DSC trace for amorphous sucrose at a moisture content of a few percent, showing a glass transition, cold crystallisation and melting.

Cold crystallisation This refers to the crystallisation process that occurs when amorphous material is heated through its glass transition temperature and is able to recrystaUise. 

Much of the current scientific knowledge discuss the mechanics to crystallisation of PET materials, primarily focusing on film, sheet and injection moulded components . Other areas focus on heat-set temperatures above the cold crystallisation temperature for PET materials... 

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